Ozone is useful for numerous applications that require a high level of oxidation. For example, ozone is useful for disinfection of drinking water and has been used for water treatment since the early 1900s. More recently, ozone has been used for semiconductor device processing. One application for ozone in semiconductor device processing is forming insulating layers on semiconductor wafers by growing insulating films or by oxidizing thin films on the wafer. For example, high deposition rate chemical vapor deposition of high quality SiO.sub.2 can be accomplished by using a TEOS/ozone process.
Another application for ozone in semiconductor device processing is for cleaning semiconductor wafers and the processing chambers of semiconductor processing equipment. Ozone is particularly useful for removing hydrocarbons from the surface of semiconductor wafers or from processing chambers. Using ozone for cleaning is advantageous because it avoids the use of dangerous chemicals which require costly disposal. In contrast, ozone does not present a toxic waste disposal problem because ozone decays to oxygen without residues.
The use of ozone in semiconductor device processing has imposed increased demands on ozone generating equipment. For semiconductor processing applications, the ozone must be very pure so that it does not introduce contaminates into the process. Some ozone generators expose elastomeric seals and electrodes to the ozone. The ozone reacts with these materials and produces contaminates that can deteriorate the elastomeric seal and some electrode materials. The use of elastomeric seals also makes leak detection difficult because such seals are designed for positive pressure.
Some ozone generators require the use of inert dopant gases such as nitrogen or carbon dioxide to increase the ozone concentration to acceptable levels. These gases are known to introduce contamination in the process. For example, when nitrogen interacts with the ozone, N.sub.x O.sub.x and nitric acid are produced if trace amounts of moisture are present in the process chamber. These chemicals corrode the process chamber and the ozone delivery system and can contaminate the wafer being processed.
In addition, ozone generators for semiconductor processing applications should be small in dimensions because clean room space in semiconductor manufacturing facilities is very expensive. Current ozone generators are large and may require substantial ancillary refrigeration equipment in order to achieve the required ozone generation rates. To reduce the size of high concentration ozone generators, some generators distribute the electrode area over a number of smaller cells. This introduces uneven gas flow to and from the various cells and, therefore, reduces ozone production efficiency. The semiconductor industry also demands high reliability and low down time of the equipment.
U.S. Pat. No. 5,364,600 describes an ozone generator that generates a high yield of ozone from an oxygen containing gas. This generator uses a spiral wire electrode formed from a single refractory metal to reduce contamination and to increase ozone yield. Wire electrodes operate at relatively high temperatures because they can not be attached directly to a heat sink. Wire electrodes have a relatively short lifetime because of the thermal stress that they experience. In addition, wire electrodes are relatively inefficient because ozone production reduces with temperature.